1. Field of the Invention
The present invention relates to a MRI (magnetic resonance imaging) apparatus and a magnetic resonance imaging method which excite nuclear spin of an object magnetically with a RF (radio frequency) signal having the Larmor frequency and reconstruct an image based on NMR (nuclear magnetic resonance) signals generated due to the excitation, and more particularly, to a magnetic resonance imaging apparatus and a magnetic resonance imaging method which acquires NMR signals with using SSFP (Steady State Free Precession).
2. Description of the Related Art
Magnetic Resonance Imaging is an imaging method which excites nuclear spin of an object set in a static magnetic field with a RF signal having the Larmor frequency magnetically and reconstruct an image based on NMR signals generated due to the excitation.
In the field of a magnetic resonance imaging, the imaging method using SSFP (Steady State Free Precession) has been known. As a typical example of high speed imaging sequence using SSFP, there is a sequence referred to TrueFISP (fast imaging with steady precession) (see, for example, U.S. Pat. No. 4,769,603).
FIG. 1 is a flowchart showing the conventional True FISP sequence.
As shown in FIG. 1, the conventional SSFP sequence such as the TrueFISP sequence applies a RF excitation pulse repeatedly at a constant and short TR (repetition time) with a same excitation angle (flip angle) α to lead magnetization in a steady state quickly. The gradient magnetic field is adjusted so that the zero-order moment (time integration) becomes zero. The gradient magnetic field in a read out axis direction is controlled so that the polarity inverts several times. As a result, an obtained echo signal has a high signal to noise ratio (SNR) and a signal intensity S depends on a relaxation time of a tissue as shown in the expression (1).S∝1/(1+T1/T2)  (1)
Note that, the expression (1) is a relational expression when an excitation angle α is 90 degrees. T1 and T2 are a longitudinal relaxation time of a tissue and a transverse relaxation time of a tissue respectively. As shown in the expression (1), the intensity S of signal obtained by the SSFP sequence depends on a relaxation time ratio T1/T2 of a tissue. Consequently, it is known that it is the most effective from the contrast viewpoint to regard a cine image of a heart as an applicable target of the SSFP sequence. In addition, the effectiveness of the SSFP sequence to imaging of the abdominal vasculature has been pointed out.
In the meanwhile, the requirements needed for the SSFP sequence include requirements with regard to a phase of RF pulse in addition to the requirement that the zero-order moment of gradient magnetic field becomes zero as described above. The simplest control requirement with regard to a phase of RF pulse is that a phase of continuous RF pulse alternates between zero degree and 180 degrees (π radian).
FIG. 2 is a diagram showing a variation of magnetization intensity by a scan under the conventional SSFP sequence.
When an angle is controlled so that each excitation angle of continuous RF pulses becomes a, and a phase is controlled so that a phase of continuous RF pulse alternates between zero degree and 180 degrees, the magnetization state alternates between the state (A) and the state (B) as shown in a vectorial representation in FIG. 2.
That is, phases of excitation pulses are controlled so that:
the excitation angle becomes α, α, α, . . . ,
the phase of excitation pulse becomes 0°, 180°, 0°, . . . , and
the state of magnetization becomes (A), (B), (A), . . . .
As shown in FIG, 2, a magnetization that reached a steady state becomes the state (A) that deviates from the static magnetic field direction by α/2. In this state (A) of magnetization, when an excitation pulse with changing the phase by 180 degrees is applied, the magnetization state changes from the state (A) to the state (fl). Moreover, in the magnetization state (B), when an excitation pulse with changing the phase by 180 degrees is applied, the magnetization state returns from the state (B) to the state (A) again.
In this way, it turns out that a steady state is maintained effectively by changing a phase of a continuous excitation pulse by 180 degrees. It is also known that the time required for transferring magnetization in thermal equilibrium to a steady state can be reduced by the foregoing phase control of an excitation pulse.
FIG. 3 is a sequence chart showing a pulse sequence derived by improving the conventional tureFISP sequence.
As shown in FIG. 3, a pulse sequence for applying a pre-pulse with an excitation angle α/2 prior to a RF excitation pulse train applied at intervals of a TR with a same excitation angle α, which is derived by improving the conventional TrueFISP sequences has been also designed. A phase angle of the pre-pulse becomes 0 degree since it differs from the phase angle 180 degrees of the first RF excitation pulse by 180 degrees.
However, the control technique of the phase angles of excitation pulses in the conventional SSFP sequence is applicable to only the case where signals are acquired from a single matter having a certain chemical shift, and can achieve an effect only in the case in which the center frequency of the excitation pulse set as an imaging condition in the apparatus side is adjusted so as to become same as a resonance frequency of a matter to be an application target. Therefore, when the center frequency of the excitation pulse set in the apparatus side is off a resonance frequency of a matter to be an application target, a magnetization rotates about the static magnetic field direction in an interval between an application of a certain excitation pulse and the application of the next excitation pulse. In such a case, a state different from a steady state as shown in FIG. 2 will be generated.
Moreover, generally various materials exist in a living body and each matter has a specific chemical shift. The typical matters in a living body include water and fat component. Consequently, A water image where signals from water are emphasized or a fat image where signals from fat are emphasized are acquired frequently. Therefore, a contrast of an image acquired by a SSFP sequence changes significantly depending on which of a water image or a fat image is acquired. In addition, a contrast of an image acquired by a SSFP sequence also changes significantly depending on whether a center frequency of an excitation pulse set on the apparatus side is adjusted to match the resonance frequency of water or the resonance frequency of fat.
For this reason, when a center frequency of an excitation pulse is set to a resonance frequency of a matter different from a matter to be imaged, there is also a possibility that a steady state of magnetization is not maintained appropriately and an image can not be obtained with a desired contrast. And even if magnetization becomes a steady state, the time until the magnetization becomes the steady state becomes long and that leads to an increase of image artifacts such as ghost and blurring.
That is, there is a problem that a steady state of a magnetization is not maintained appropriately and an image with a satisfactory contrast can not obtained in case where a center frequency of an excitation pulse set as an imaging condition is not adjusted properly and becomes different from a resonance frequency of a matter to be a imaging target if the control technique of a phase angle of an excitation pulse in the conventional SSFP sequence is used.
FIG. 4 is a diagram showing a variation of magnetization in a matter under a method for controlling a phase angle of an excitation pulse based on the conventional SSFP sequence.
FIG. 4 is a diagram in which a transverse magnetization of a matter to be imaged in the XY direction is viewed from a static magnetic field direction in a system that rotates with a same frequency as the center frequency of an excitation pulse with respect to the laboratory system.
When a transverse magnetization of a matter turned to the (n) position shown in FIG. 4 by application of the n-th RF excitation pulse, the transverse magnetization rotates by 2π·Δf·TR immediately before application of the (n+1)-th RF excitation pulse in case where a center frequency of a RF excitation pulse differs from a resonance frequency of a matter to be imaged by Δf [Hz]. Here, TR denotes a repetition time of a RF excitation pulse.
A phase angle of the (n+1)-th RF excitation pulse differs from a phase angle of the n-th RF excitation pulse by 160 degrees. Therefore, the transverse magnetization rotates into the (n+1) position shown in FIG. 4 immediately after the application of the (n+1)-th RF excitation pulse.
As described above, it turns out that a size of a transverse magnetization changes with each excitation and a steady state of magnetization is not maintained by the control technique of a phase angle of an excitation pulse based on the conventional SSFP sequence. And if a steady state of magnetization is not maintained. well, a signal intensity fluctuates and that leads to not only appearance of artifacts such as ghost and blurring but also change of a contrast itself of an image.
In addition, problems in the control technique of a phase angle of an excitation pulse in the conventional SSFP sequence include a possibility that a steady state of magnetization breaks due to a variable magnetic field in a static magnetic field. Representative examples of variable magnetic field include a B0 magnetic field, having a uniform spatial distribution, caused by an eddy current generated with a drive of a gradient magnetic field pulse and a B0 magnetic field generated by coupling of a gradient magnetic field coil or a shim coil and the static magnetic field magnet. When these B0 magnetic fields are generated, a magnetization is to rotate in the static magnetic field direction. As a result, a phase shift of magnetization, equivalent to that due to setting the center frequency of a RF excitation pulse to an inappropriate frequency, occurs.
That is, a magnetization starts a phase rotation around the static magnetic field in each TR due to a B0 magnetic field and the consistency in phase with a RF excitation pulse applied consecutively breaks up. Consequently, this leads to a change of a contrast of an image and generation of artifacts. This means there is a problem that a sufficient steady state of a magnetization can not be obtained in case where the influence of a B0 magnetic field is not negligible under the conventional control technique of a phase of an excitation pulse.